The hydraulically driven fan is mounted on a construction machine to blow into a radiator and an oil cooler.
Patent Reference 1 describes a control device of a hydraulically driven fan provided with an engine, a variable displacement hydraulic pump driven by the engine, a hydraulic motor driven by hydraulic oil supplied from the variable displacement hydraulic pump, a hydraulically driven fan driven by the hydraulic motor, a control valve for controlling a swash plate of the variable displacement hydraulic pump, and a controller for sending an electric current command to the control valve. And, when an engine water temperature, a hydraulic oil temperature and a number of engine rotations are inputted into the controller, the controller calculates a target fan speed corresponding to the engine water temperature, the hydraulic oil temperature and the number of engine rotations, generates an electric current command necessary to set the target fan speed, and sends it to the control valve to match the actual fan speed with the target fan speed by open-loop control.
Patent Reference 2 describes a control device of a hydraulically driven fan configured in the same manner as in Patent Reference 1. In Patent Reference 2, to prevent peak pressure from occurring inside an oil passage ranging from a hydraulic pump to a hydraulic motor or an outlet pressure of the hydraulic motor from causing pressure hunting, the electric current command is sent to the control valve to retain a minimum fan speed for a prescribed time at the time of starting the engine and subsequently to gradually increase the fan speed until reaching the target fan speed. Patent Reference 2 controls the fan speed by feedback control or PI control (proportional integral control).